Silicon carbide semiconductor device and method for producing the same

ABSTRACT

A silicon carbide semiconductor device ( 90 ), includes: 1) a silicon carbide substrate ( 1 ); 2) a gate electrode ( 7 ) made of polycrystalline silicon; and 3) an ONO insulating film ( 9 ) sandwiched between the silicon carbide substrate ( 1 ) and the gate electrode ( 7 ) to thereby form a gate structure, the ONO insulating film ( 9 ) including the followings formed sequentially from the silicon carbide substrate ( 1 ): a) a first oxide silicon film (O) ( 10 ), b) an SiN film (N) ( 11 ), and c) an SiN thermally-oxidized film (O) ( 12, 12   a,    12   b ). Nitrogen is included in at least one of the following places: i) in the first oxide silicon film (O) ( 10 ) and in a vicinity of the silicon carbide substrate ( 1 ), and ii) in an interface between the silicon carbide substrate ( 1 ) and the first oxide silicon film (O) ( 10 ).

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.13/299,136, filed on Nov. 17, 2011, which is a Divisional of U.S.application Ser. No. 11/991,249, filed on Feb. 29, 2008, now U.S. Pat.No. 8,222,648, which is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application No. PCT/JP2006/316795, filed on Aug. 22, 2006,which in turn claims the benefit of Japanese Application No.2005-247175, filed on Aug. 29, 2005, the disclosures of whichApplications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a silicon carbide semiconductor devicehaving a metal-insulator-semiconductor (MIS) structure with highreliability and also relates to a method for producing the same.

BACKGROUND ART

In terms of a power device, between an on resistance and a reverseblocking voltage, there is a trade off relation which is specified, inprinciple, by a forbidden band gap. Therefore, in the current Si powerdevice, obtaining high performance beyond the theoretical limitdetermined by the forbidden band of Si is of difficulty. However, makingthe power device by a semiconductor material having a wide forbiddenband gap can greatly relieve the conventional trade off relation,thereby realizing a device which is remarkably improved in at least oneof the on resistance and the reverse blocking voltage.

With the temperature so increased as to boomingly generate aelectron-positive hole pair by thermal excitation, a semiconductor isunable to distinguish p type area from n type area or to control carrierdensity, making it difficult to operate the device. In the case of an Sisemiconductor having a forbidden band gap of 1.12 eV, generation of theelectron-positive hole pairwise is intensified from around 500 K (=227°C.), therefore, a practical upper limit temperature as a semiconductordevice is 180° C. on the premise of a continuous operation. Making asemiconductor device (not limited to a power device) using a wideforbidden band material will greatly increase an operating temperaturearea (for example, more than or equal to 300° C.), greatly wideningapplication of the semiconductor device.

The silicon carbide (hereinafter denoted “SiC”) semiconductor under thepresent invention is one of the wide forbidden band semiconductormaterials capable of improving performance. Recently, with thedevelopment of single crystal substrate, a wafer (3C, 6H, 4H) featuringa comparatively good quality and having a diameter of more than or equalto three inches is commercially available. SiC has a forbidden band gap,specifically, 3C crystal system having 2.23 eV, 6H crystal system having2.93 eV, and 4H crystal system having 3.26 eV, each sufficiently widerthan those of Si. Compared with other wide forbidden bandsemiconductors, SiC is chemically stable extremely and mechanicallyrigid. With the SiC semiconductor, forming of pn junction, controllingof impurity density and selectively forming of impurity area arepossible in a method like that for producing Si semiconductor.

In addition, SiC is especially outstanding over other wide forbiddenband semiconductors. Specifically, like Si, SiC is a uniquesemiconductor capable of generating oxide silicon (SiO₂) by thermaloxidizing, which is an advantage. With this, it is expected that anormally-off type MOS drive device, for example, a power MOSFET(Metal-Oxide-Semiconductor Field Effect Transistor) or power IGBT(Insulated Gate Bipolar Transistor) can be realized by the SiC, makingvarious companies vigorously develop the SiC.

Realizing the MOS drive SiC device, however, may cause various problems.

Among other things, a drastic improvement in reliability of gate oxidefilm is the greatest issue. Primarily, an SiC thermally-oxidized filmhas the following features: (1) an energy barrier against a conductiveelectron of SiO₂/SiC interface is, in principle, smaller than that of anSi thermally-oxidized film, and (2) a considerable amount of C (carbon)as a remnant is included in SiO₂. Thus, it was expected that the SiCthermally-oxidized film, in principle, causes more leak current than theSi thermally-oxidized film and finds difficulty in bringing about ashigh reliability (root cause) as that brought about by the Sithermally-oxidized film. However, reliability of an actual SiCthermally-oxidized film is below the above expectation, causing furtherdeterioration.

Reasons therefor are to be explained. The Si device is known for that anSi thermally-oxidized film formed by thermally oxidizing a substratehaving a surface with crystal imperfection (dislocation and the like)causes an insulation breakdown in a low electric field or is remarkablydecreased in time dependent dielectric breakdown (TDDB) lifetime. TheSiC thermally-oxidized film may cause the like effect. In “Tanimoto etal., Extended Abstracts (The 51st Spring Meeting, Tokyo University ofTechnology, 2004); The Japan Society of Applied Physics and RelatedSocieties, p. 434, Lecture No. 29p-ZM-5 (hereinafter referred to as“non-patent document 1”)”, the present inventors (Satoshi Tanimoto isthe present inventor) reported the following: TDDB lifetime of a gateoxide film of a power MOSFET having a practical area depends on defectattributable to a great amount of dislocations on the surface of the SiCsubstrate used. As a result, compared with the Si thermally-oxidizedfilm (having no same defect), the SiC thermally-oxidized film has TDDBlifetime decreased by more than or equal to two-digit.

Use of a layered (gate) insulating film may solve the above reliabilityproblems of the SiC thermally-oxidized film, although not so muchreports have been made thereon. Among the above, an ONO gate insulatingfilm is the most desirable and practical. In “ONO”, “O” denotes SiO₂film (oxide silicon film) and “N” denotes Si₃N₄ film (silicon nitridefilm. Otherwise, denoted “SiN film” for short).

In “IEEE Transactions on Electron Devices, Vol. 46, (1999). p. 525”(hereinafter referred to as “non-patent document 2”), L. A. Lipkin etal. studied reliability of a metal-insulator-semiconductor (MIS)structure of a gate electrode having the following structure:

Between i) an n⁺ type 4H—SiC substrate (having a surface where an n⁻type epitaxial layer is grown) and ii) a Mo/Au gate electrode, an ONOgate insulating film is sandwiched which includes:

-   -   1) SiC thermally-oxidized film,    -   2) an SiN film produced by an LPCVD (Low Pressure Chemical Vapor        Deposition), and    -   3) an SiO₂ film formed by thermally oxidizing the surface of the        above SiN film in 2).

The study by L. A. Lipkin et al. has obtained a maximum insulationbreakdown strength BEox=about 13.1 MV/cm (SiO₂ converted), and a maximumstress current strength BJox=about 0.25 mA/cm².

Herein, the superscript “+” and the superscript “−” on “n” or “p” eachdenoting conductivity (negative or positive) of the semiconductordenote, respectively, high density and low density.

Meanwhile, X. W. Wang et al., in “IEEE Transactions on Electron Devices,Vol. 47, (2000) p. 458” (hereinafter referred to as “non-patent document3”) discloses an evaluation on reliability of an MIS structure where anONO gate insulating film formed by thermal oxidizing of a surface ofSiO₂/SiN films layered by a JVD (jet vapor deposition) is sandwichedbetween a 6H—SiC substrate and an Al gate electrode, to thereby obtainBEox=about 12.5 MV/cm (SiO₂ converted) and BJox=3 mA/cm².

However, the above two ONO gate insulating films according to thenon-patent document 2 and the non-patent document 3 each are lessreliable than the SiC thermally-oxidized film. Actually, in “MaterialScience Forum, Vols. 433-436, (2003) p. 725” (hereinafter referred to as“non-patent document 4”), the present inventors Satoshi Tanimoto et al.report an accomplishment of BEox=13.2 MV/cm and BJox>100 mA/cm² by usinga MOS structure including a thermally-oxidized film of 4H—SiC substrate.As obvious by comparison with the result of the non-patent document 4,the above two ONO gate insulating films according to the non-patentdocument 2 and the non-patent document 3 are less reliable than the SiCthermally-oxidized film obtained by the present inventors in terms ofBEox and BJox.

Under the above background, recognizing potentiality of the ONO gateinsulating film, the present inventors studied a method for applying theONO gate insulating film to a structure or production processes of anactual power MOS device. By the following operations, the presentinventors successfully accomplished BEox=21 MV/cm and BJox>10A/cm² whichare far better in performance than those of the above two ONO gateinsulating films according to the non-patent documents 2 and 3 and theconventional SiC thermally-oxidized film in the non-patent document 1:

1) Between a polycrystalline silicon gate electrode and an SiCsubstrate, sandwiching the ONO insulating film where i) an SiCthermally-oxidized film ii) CVD silicon nitride film and iii) athermally-oxidized film of the CVD silicon nitride film in ii) aresequentially layered.

2) Providing the polycrystalline silicon thermally-oxidized film and thesilicon nitride sideface thermally-oxidized film, respectively, on asideface of the gate electrode and a sideface of the silicon nitridefilm.

Refer to “Satoshi Tanimoto et al., Material Science Forum, Vols.483-485, (2005) p. 677”, hereinafter referred to as “non-patent document5”. This ONO insulating film structure has TDDB lifetime (=chargequantity passing per unit area until insulation breakdown) ofQ_(BD)=about 30 C/cm² which is at least two-digit higher than that ofthe SiC thermally-oxidized film, and is substantially equivalent to thatof a thermally-oxidized film on a non-defect single crystal Sisubstrate.

DISCLOSURE OF THE INVENTION

According to the conventional technology (non-patent document 5), theTDDB lifetime of the ONO gate insulating film is greatly improved tosuch an extent as to reach that of the Si thermally-oxidized film, butthe ONO gate insulating film is not necessarily sufficient for a longtime operation at a temperature higher than a practical upper limittemperature of the Si (MOS) device, therefore requiring furtherimprovement in TDDB lifetime.

It is an object of the present invention to provide a silicon carbidesemiconductor device with an improved practical upper limit temperature,and a method for producing the same.

According to a first aspect of the present invention, there is provideda silicon carbide semiconductor device, comprising: 1) a silicon carbidesubstrate; 2) a gate electrode made of polycrystalline silicon; and 3)an ONO insulating film sandwiched between the silicon carbide substrateand the gate electrode to thereby form a gate structure, the ONOinsulating film including the followings formed sequentially from thesilicon carbide substrate: a) a first oxide silicon film (O), b) an SiNfilm (N), and c) an SiN thermally-oxidized film (O), wherein nitrogen isincluded in at least one of the following places: i) in the first oxidesilicon film (O) and in a vicinity of the silicon carbide substrate, andii) in an interface between the silicon carbide substrate and the firstoxide silicon film (O).

According to a second aspect of the present invention, there is provideda method for producing the silicon carbide semiconductor deviceaccording to the first aspect, wherein the first oxide silicon film (O)is formed by a heat treatment in an oxidized nitrogen (NOx) gasatmosphere in a period after forming of a precursor oxide silicon filmand before depositing of the SiN film (N).

According to a third aspect of the present invention, there is provideda method for producing the silicon carbide semiconductor deviceaccording to the first aspect, wherein the first oxide silicon film (O)is formed by a reoxidization in an oxidized nitrogen (NOx) gasatmosphere in a period after forming of a precursor oxide silicon filmand before depositing of the SiN film (N).

According to a fourth aspect of the present invention, there is provideda method for producing the silicon carbide semiconductor deviceaccording to the first aspect, wherein the first oxide silicon film (O)is formed by thermally oxidizing a surface of the silicon carbidesubstrate in an oxidized nitrogen (NOx) gas atmosphere.

According to a fifth aspect of the present invention, there is provideda method for producing the silicon carbide semiconductor deviceaccording to the first aspect, wherein the first oxide silicon film (O)is formed by the following sequential operations: 1) forming a thinoxide silicon film by one of the following sub-operations: i) a heattreatment in an oxidized nitrogen (NOx) gas atmosphere in a period afterforming of a precursor oxide silicon film and before depositing of theSiN film (N), ii) a reoxidization in the oxidized nitrogen (NOx) gasatmosphere in the period after forming of the precursor oxide siliconfilm and before depositing of the SiN film (N), and iii) thermallyoxidizing a surface of the silicon carbide substrate in the oxidizednitrogen (NOx) gas atmosphere, and 2) depositing, on the thin oxidesilicon film, another oxide silicon film which is formed by an operationother than the thermal oxidizing.

The other features, advantages and benefits of the present inventionwill become apparent from the following description in conjunction withthe following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of essential parts of a semiconductordevice, according to the first embodiment of the present invention.

FIG. 2 is a cross sectional view of the production operation of thesemiconductor device, according to the first embodiment of the presentinvention.

FIG. 3 is a cross sectional view of the production operation of thesemiconductor device, according to the first embodiment of the presentinvention.

FIG. 4 is a cross sectional view of the production operation of thesemiconductor device, according to the first embodiment of the presentinvention.

FIG. 5 is a cross sectional view of the production operation of thesemiconductor device, according to the first embodiment of the presentinvention.

FIG. 6 is a cross sectional view of the production operation of thesemiconductor device, according to the first embodiment of the presentinvention.

FIG. 7 shows characteristics of a gate insulating film of thesemiconductor device, according to the first embodiment of the presentinvention.

FIG. 8 is a cross sectional view of essential parts of the semiconductordevice, according to the second embodiment of the present invention.

FIG. 9 is a cross sectional view of the production operation of thesemiconductor device, according to the second embodiment of the presentinvention.

FIG. 10 is a cross sectional view of the production operation of thesemiconductor device, according to the second embodiment of the presentinvention.

FIG. 11 is a cross sectional view of the production operation of thesemiconductor device, according to the second embodiment of the presentinvention.

FIG. 12 is a cross sectional view of essential parts of a semiconductordevice, according to the third embodiment of the present invention.

FIG. 13 is a cross sectional view of the production operation of thesemiconductor device, according to the third embodiment of the presentinvention.

FIG. 14 is a cross sectional view of the production operation of thesemiconductor device, according to the third embodiment of the presentinvention.

FIG. 15 is a cross sectional view of the production operation of thesemiconductor device, according to the third embodiment of the presentinvention.

FIG. 16 is a cross sectional view of the production operation of thesemiconductor device, according to the third embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, referring to the drawings, various embodiments of thepresent invention are to be set forth in detail. Unless otherwisespecified, the term “substrate” is an SiC substrate on which any of anepitaxial layer, other film and other electrode is formed.

In the following drawings, the same or similar parts are to be denotedby the same or similar reference numerals or signs, and the descriptionthereof is, as the case may be, simplified or omitted. The drawings areschematic, and therefore i) thickness relative to plan-view dimension orii) thickness ratio and the like of each layer is, as the case may be,not actual. Therefore, specific thickness or dimension should bedetermined referring to the following explanation. Moreover, relativedimension or ratio between the drawings is, as the case may be, notactual.

First Embodiment Structure

FIG. 1 is a cross sectional view of an essential part of a siliconcarbide semiconductor device 90 having an MIS structure (capacitor)including a high-reliability ONO layered film, according to a firstembodiment of the present invention, where MIS denotesMetal-Insulator-Semiconductor.

In FIG. 1, there is provided an n⁺ type 4H—SiC epitaxial substrate 1having a high impurity density (nitrogen>1×10¹⁹/cm³). On an uppersurface of the n⁺ type 4H—SiC epitaxial substrate 1, an n⁻ typeepitaxial layer is grown homo-epitaxially. Substrates having otherstructural systems such as 6H, 3C and 15R can be used, where H denoteshexagonal system, C denotes cubic system and R denotes rhombohedralsystem.

Moreover, substrates having the following structures can be used:

i) p type epitaxial layer or p type SiC substrate, or

ii) p type or n type epitaxial layer grown on semi-insulating SiCsubstrate.

On the SiC epitaxial substrate 1, there is disposed a field insulatingfilm 3 having a thickness of more than or equal to several 100 nm.

The field insulating film 3 includes i) a thin lower insulating film 4formed by thermally-oxidizing the SiC substrate (technically, epitaxiallayer) and ii) a thick upper insulating film 5 formed by an operation(for example, LPCVD [Low Pressure Chemical Vapor Deposition] and thelike) other than the thermal oxidizing of the SiC substrate and layeredon the lower insulating film 4. The field insulating film 3 is formedwith a gate window 6 which is open.

A gate electrode 7 made of polycrystalline Si is provided in such amanner as to cover the gate window 6. At least a sideface of thepolycrystalline Si gate electrode 7, there is formed a polycrystallineSi thermally-oxidized film 8 grown by thermal oxidizing. Between the SiCepitaxial substrate 1 (base of the gate window 6) and the gate electrode7, an ONO gate insulating film 9 having a 3-layer structure issandwiched. Between the SiC epitaxial substrate 1 and thepolycrystalline Si gate electrode 7, the ONO gate insulating film 9includes: i) a first oxide silicon film 10 (O), ii) an SiN film 11 (N)and iii) an SiN thermally-oxidized film 12 (O: second oxide siliconfilm), which are arranged sequentially from the SiC epitaxial substrate1. The semiconductor device 90 has a gate structure sandwiching the ONOgate insulating film 9.

The lowermost part (SiC epitaxial substrate 1 side) of the 3-layerstructure of the ONO gate insulating film 9, namely, the first oxidesilicon film 10 includes N (nitrogen) in at least one of the followingplaces: i) in an interface between the first oxide silicon film 10 andthe SiC epitaxial substrate 1 and ii) in the vicinity of the interface.The first oxide silicon film 10 is localized around an area of the gatewindow 6. The oxide silicon film 10 has a thickness from 3.5 nm to 25nm, especially, thickness from 4 nm to 10 nm giving an extremelypreferable result.

The oxide silicon film 10 is formed by the following sequentialoperations:

i) thermally-oxidizing the surface of the SiC epitaxial substrate 1, and

ii) heat treatment or reoxidization in an oxidized nitrogen (NOx) gasatmosphere.

For forming the oxide silicon film 10, otherwise, the surface of the SiCepitaxial substrate 1 may be thermally oxidized directly by the oxidizednitrogen gas (NOx).

In case the SiC thermally-oxidized film cannot be used due to structuralrestriction of the semiconductor device 90, the oxide silicon film 10may be formed by the following operation: an SiO₂ film deposited bychemical vapor deposition (CVD) is subjected to the heat treatment orreoxidization in the oxidized nitrogen (NOx) gas atmosphere.

Of the 3-layer structure of the ONO gate insulating film 9, the SiN film11 is an intermediate layer (═N) deposited by LPCVD and the like, andthe SiN thermally-oxidized film 12 (namely, SiO₂ film) grown byoxidizing the surface of the SiN film 11 is an uppermost layer (═O). TheSiN film 11 and the SiN thermally-oxidized film 12 are formed to extendon the field insulating film 3. The SiN film 11 and the SiNthermally-oxidized film 12 have thickness, for example, 53 nm and 5 nmrespectively. On an outer edge sideface of the SiN film 11, there isdisposed a thin SiN sideface thermally-oxidized film 13 (namely, SiO₂film) grown by thermally-oxidizing the SiN film 11. When viewed on aplan, the polycrystalline Si gate electrode 7 has an outer edge Gdisposed inside an outer edge N of the SiN film 11.

On the gate electrode 7 and the field insulating film 3, an interlayerdielectric film 14 (for short, otherwise referred to as “ILD film 14”)is formed. A gate contact window 15 is so open in the ILD film 14 as topass through the gate electrode 7. Instead of being in the gate window 6as shown in FIG. 1, the gate contact window 15 may have such a structureas to be disposed on the gate electrode 7 extending on the fieldinsulating film 3. Via the gate contact window 15, an inner wiring 16connects the gate electrode 7 to other circuit element(s) on the samesubstrate or to an outer circuit.

On a backface of the SiC epitaxial substrate 1, there is disposed anohmic contact electrode 17 having an extremely low resistance. The ohmiccontact electrode 17 is formed by the following sequential operations:i) a contact metal such as Ni is vacuum deposited on a back of the SiCepitaxial substrate 1, ii) alloying the thus obtained with SiC by arapid heat treatment at a temperature lower than the thermal oxidizingtemperature for the oxide silicon film 10 (namely, SiCthermally-oxidized film) of the ONO gate insulating film 9, for example,1000° C. relative to the thermal oxidizing at 1100° C.

<Production Method>

Then, referring to FIG. 2( a) to FIG. 6( i), a method for producing theMIS structure (see FIG. 1) including the ONO gate insulating film 9 isto be explained, according to the first embodiment of the presentinvention.

Operation (a): A (0001) Si face 8° off cut n⁺ type 4H—SiC epitaxialsubstrate 1 having an upper surface where a high-quality n⁻ typeepitaxial layer is grown is sufficiently cleaned by an RCA cleaning andthe like.

Herein, the RCA cleaning is a semiconductor substrate cleaning methodcombining H₂O₂+NH₄OH mix solution cleaning and H₂O₂+HCl mix solutioncleaning.

Then, after dry oxidizing, as shown in FIG. 2( a), on the upper surfaceof the SiC epitaxial substrate 1, the field insulating film 3 is formedhaving the thin lower insulating film 4 and the thick upper insulatingfilm 5. The lower insulating film 4 is an SiC thermally-oxidized filmformed by dry oxidizing of the surface of the SiC epitaxial substrate 1in an oxygen atmosphere, and has a thickness about 10 nm. Meanwhile, theupper insulating film 5 is formed by an operation other than the thermaloxidizing and has a certain thickness, for example, the upper insulatingfilm 5 is an SiO₂ film formed by an atmospheric pressure CVD usingoxygen and silane and has thickness of 400 nm. The thermal oxidizing ofthe lower insulating film 4 is not limited to the dry oxidizing, otheroxidizing operations such as i) wet oxidizing and ii) using otheroxidizing gas are also usable. The lower insulating film 4 has athickness less than 50 nm, preferably 5 nm to 20 nm. As described above,forming of the upper insulating film 5 may be after the growth of thelower insulating film 4 on the surface of the SiC epitaxial substrate 1.Contrary to this, after the forming of the upper insulating film 5followed by the thermal oxidizing, the lower insulating film 4 may begrown between the SiC epitaxial substrate 1 and the upper insulatingfilm 5. In FIG. 2( a), there is provided a first transient SiCthermally-oxidized film 201 which is automatically formed on thebackface of the SiC epitaxial substrate 1 in the forming of the lowerinsulating film 4. The first transient SiC thermally-oxidized film 201is used for effectively removing a significantly deep-grinding damagelayer on the backface of the SiC epitaxial substrate 1.

Operation (b): Then, a photoresist mask {not shown in FIG. 2( b)} isformed on the surface of the SiC epitaxial substrate 1 byphotolithography, followed by wet etching of the SiC epitaxial substrate1 with a buffered hydrofluoric acid solution (BHF=NH₄F+HF mix solution),to thereby form the gate window 6 in a certain position of the fieldinsulating film 3, as shown in FIG. 2( b). The above wet etching removesthe first transient SiC thermally-oxidized film 201. For forming thegate window 6 that is fine, a dry etching such as reactivity ion etching(RIE) and the like with CF₄ gas plasma can be used. In this case,however, at first, the dry-etching is implemented to thereby leave thefield insulating film 3 having a thickness of several 10 nm, then thewet etching using the above BHF acid solution replaces the dry-etching.Using the dry etching only for the through-opening of the gate window 6may damage the SiC surface by plasma, thus deteriorating the gateinsulating film 9 to be formed in the subsequent operation (c). Afterthe etching of the gate window 6, the photoresist mask {not shown inFIG. 2( b)} is to be stripped off {see FIG. 2( b)}.Operation (c): Then, the SiC epitaxial substrate 1 is again cleaned bythe RCA cleaning. At the final step of the RCA cleaning, for removingthe chemically oxidized film generated on the surface of the open partby the RCA cleaning, the SiC epitaxial substrate 1 is dipped in the BHFacid solution for 5 sec. to 10 sec., followed by a complete rinsing ofthe BHF acid solution with a super pure water, and still followed bydrying. Then, any of the following operation (c1) to operation (c4) isto be implemented, so as to form, on the surface of the epitaxial layerat the base of the gate window 6, the oxide silicon film 10 whichincludes N (nitrogen) in at least one of the following places: i) in aninterface between the first oxide silicon film 10 and the SiC epitaxialsubstrate 1 and ii) in the vicinity of the interface {see FIG. 2( c)}.Operation (c1): At first, the SiC epitaxial substrate 1 isthermally-oxidized (for example, dry oxidizing at a temperature 1160°C.), to thereby grow an SiC thermally-oxidized film on the surface ofthe epitaxial layer at the base of the gate window 6.

Then, the SiC epitaxial substrate 1 is subjected to a heat treatment (orreoxidization) in an oxidized nitrogen (NOx) gas atmosphere, to therebyconvert the SiC thermally-oxidized film to the oxide silicon film 10.Examples of the oxidized nitrogen gas (NOx) for the heat treatment (orreoxidization) include i) N₂O (nitrous oxide), ii) NO (nitrogenmonoxide), iii) NO₂ (nitrogen dioxide), iv) a mixture of at least two ofi) to iii), v) a diluted gas of any one of i) to iii), and vi) a dilutedgas of the mixture iv), each to be properly used. The heat treatment (orreoxidization) temperature can be selected from a range of 1000° C. to1400° C. In view of shorter treatment time and lower treatment devicecost, however, 1100° C. to 1350° C. is practical and preferable.

Operation (c2): The surface of the SiC epitaxial substrate 1 issubjected to a direct thermal oxidizing by the oxidized nitrogen gas(NOx), to thereby form the oxide silicon film 10. Examples of theoxidized nitrogen gas (NOx) for the direct thermal oxidizing include i)N₂O (nitrous oxide), ii) NO (nitrogen monoxide), iii) NO₂ (nitrogendioxide), iv) a mixture of at least two of i) to iii), v) a diluted gasof any one of i) to iii), and vi) a diluted gas of the mixture iv), eachto be properly used. The heat treatment (or reoxidization) temperaturecan be selected from a range of 1000° C. to 1400° C. In view of shortertreatment time and lower treatment device cost, however, 1000° C. to1350° C. is practical and preferable.Operation (c3): At first, by an operation other than the thermaloxidizing of SiC, an SiO₂ film having a certain thickness is formed onthe surface of the SiC epitaxial substrate 1. Examples of the operationfor forming the SiO₂ film include i) chemical vapor deposition (CVD)using oxygen and silane (SiH₄) as raw materials, and ii) other formingoperations. Specifically, at first, a thin polycrystalline Si film or athin amorphous Si film is deposited on the entire face of the SiCepitaxial substrate 1, followed by a complete dry oxidizing (thermaloxidizing) at 900° C., to thereby form the SiO₂ film.

Then, the SiC epitaxial substrate 1 is subjected to a heat treatment inan oxidized nitrogen (NOx) gas atmosphere (or reoxidization), to therebyconvert the SiO₂ deposit film to the oxide silicon film 10. Examples ofthe oxidized nitrogen gas (NOx) for the heat treatment (orreoxidization) include i) N₂O (nitrous oxide), ii) NO (nitrogenmonoxide), iii) NO₂ (nitrogen dioxide), iv) a mixture of at least two ofi) to iii), v) a diluted gas of any one of i) to iii), and vi) a dilutedgas of the mixture iv), each to be properly used. The heat treatment (orreoxidization) temperature can be selected from a range of 1000° C. to1400° C. In view of shorter treatment time and lower treatment devicecost, however, 1100° C. to 1350° C. is practical and preferable.

The heat treatment (or reoxidization) in the NOx atmosphere increasesdensity of the oxide silicon film 10, as the case may be, presentingseveral % to several 10% decrease in film thickness. The above increaseddensity decreasing the film thickness can further improve reliability.

Operation (c4): At first, by the operation in any of the operation (c1)to the operation (c3), the oxide silicon film (intermediate) including Ni) in an interface between the first oxide silicon film 10 and the SiCepitaxial substrate 1 or ii) in the vicinity of the interface is soformed as to have a thickness thinner than a certain thickness. Then, onthe oxide silicon film (intermediate), the SiO₂ film is deposited by anoperation {for example, chemical vapor deposition (CVD) with oxygen andsilane (SiH₄) as raw materials} other than the thermal oxidizing of SiCto such an extent as to obtain the certain thickness, to thereby formthe oxide silicon film 10 in combination with the oxide silicon film(intermediate).

In a nutshell, there are various operations for forming the oxidesilicon film 10, therefore, any of the operation (c1) to operation (c4)can bring about the effect of the present invention. The oxide siliconfilm 10 and the forming method therefor are essential for improving TDDB(Time Dependent Dielectric Breakdown) lifetime of the ONO gateinsulating film 9.

The operations (c1) to (c4) for forming the oxide silicon film 10 arecommon in implementing the heat treatment (including oxidization andreoxidization) at a high temperature using the oxidized nitrogen gas(NOx). In terms of setting the heat treatment temperature, however,there is an essential point to be summarized as below:

Preferably, the heat treatment temperature should be so set as to behigher than any heat treatment temperatures for all operations after theforming of the oxide silicon film 10. Under the present invention, theONO gate insulating film 9 which is formed without meeting the abovepreferable heat treatment temperature, as the case may be, has the TDDBlifetime decreased than expected or deteriorates the interface betweenthe oxide silicon film 10 and the SiC.

Then, with the oxide silicon film 10 formed at the base of the gatewindow 6, the SiN film 11 (=second layer of the ONO gate insulating film9) is deposited on the entire surface of the SiC epitaxial substrate 1by the LPCVD using SiH₂Cl₂ and O₂. Immediately after the depositing, theSiC epitaxial substrate 1 is subjected to a pyrogenic oxidization at950° C., to thereby grow on the surface of the SiN film 11 the SiNthermally-oxidized film 12 (=third layer of the ONO gate insulating film9) having a certain thickness. FIG. 2( c) shows a cross sectionalstructure of the SiC epitaxial substrate 1 at this step in theoperation.

The SiC epitaxial substrate 1 has a backface which is a transient oxidesilicon film 202 automatically formed in the operation for forming theoxide silicon film 10. Likewise, a transient SiN film 203 and atransient SiN thermally-oxidized film 204 are automatically formed onthe backface of the SiC epitaxial substrate 1 by, respectively, thedepositing of the SiN film 11 and the growing of the SiNthermally-oxidized film 12. Like the first transient SiCthermally-oxidized film 201 {see FIG. 2( a)}, the transient oxidesilicon film 202 effectively removes the grinding damage layer on thebackface of the SiC epitaxial substrate 1, in addition, featuring anessential function to protect the backface of the SiC epitaxialsubstrate 1 from dry etching damage which may be caused when thepolycrystalline Si on the backface is removed, to be explained in thesubsequent operations. Moreover, the transient oxide silicon film 202can decrease contact resistance of the oxide silicon film 10 (backfaceelectrode 10).

Herein, a more detailed drawing of the cross sectional structure formedthrough any of the operation (c3) and operation (c4) shows the oxidesilicon film 10 riding on the field insulating film 3, as shown in FIG.3( c′). FIG. 3( c′) is substantially the same as FIG. 2( c) in that thebase of the gate window 6 is covered with the oxide silicon film 10, andtherefore, FIG. 3( c′) can be schematically drawn like FIG. 2( c) forexplaining the subsequent operations, causing no erroneous descriptionof the essentials of the production method under the present invention.For convenience sake, the subsequent operations are to be set forthbased on the structure of FIG. 2( c) only.

Herein, under the present invention, in the operation (c1), the SiCepitaxial substrate 1 is at first thermally oxidized, to thereby growthe SiC thermally-oxidized film on the surface of the epitaxial layer atthe base of the gate window 6, in the operation (c3), the SiO₂ filmhaving a certain thickness is at first formed on the surface of the SiCepitaxial substrate 1 by the operation other than the thermal oxidizingof SiC. The SiC thermally-oxidized film in the operation (c1) and theSiO₂ film in the operation (c3) are each referred to as “precursor oxidesilicon film.” Then, in the heating in the oxidized nitrogen (NOx) gasatmosphere; a new thermally-oxidized film is formed between theprecursor oxide silicon film (such as the SiC thermally-oxidized film)and the SiC substrate, where such an operation is referred to as“reoxidization,” meanwhile, when the new thermally-oxidized film is notformed, such an operation is referred to as “heat treatment.” The abovedefinitions of the reoxidization and heat treatment are to be usedlikewise hereinafter.

Operation (d): Then, by the low pressure CVD (growth temperature of 600°C. to 700° C.) using silane as raw material, the polycrystalline siliconfilm having a thickness of 300 nm to 400 nm is formed on the entiresurface and backface of the SiC epitaxial substrate 1. Then, P(phosphor) is added to the polycrystalline silicon film by a known heatdiffusing method (treatment temperature of 900° C. to 950° C.) usingphosphor chlorate (POCl₃) and oxygen, to thereby bring aboutconductivity. For bringing about p type conductivity, B (boron) can beadded, replacing the P (phosphor).

Then, a photoresist mask is formed on the surface of the SiC epitaxialsubstrate 1 by photolithography, then, the polycrystalline Si film, theSiN thermally-oxidized film 12 and the SiN film 11 are sequentiallyetched by reactivity ion etching (RIE) using SF₆, to therebysubstantially define (pre-defining) outer edges of i) thepolycrystalline Si gate electrode 7, ii) the SiN thermally-oxidized film12 and iii) the SiN film 11. With the above operation, a nonessentialpart of the ON layer (O: SiN thermally-oxidized film 12, N: SiN film 11)is etched (removed) accurately in a self-aligning manner by thephotoresist mask same as that for the polycrystalline Si gate electrode7, such that the ON layer can share the outer edge.

Then, the thus used photoresist mask is completely removed, then dryetching of the backface is implemented while protecting the surface ofthe SiC epitaxial substrate 1 by applying again, to the entire surfaceof the SiC epitaxial substrate 1, the resist material (photoresistallowed) having a thickness more than or equal to 1 μm, then, thepolycrystalline Si film (not shown), the transient SiNthermally-oxidized film 204, and the transient SiN film 203 {see FIG. 2(c)} which three are deposited on the backface side are sequentiallyremoved, and then, the resist material for surface protection isstripped off, to thereby form the cross sectional structure shown inFIG. 4( d).

Operation (e): Then, the SiC epitaxial substrate 1 is again subjected tothe RCA cleaning, followed by purifying and drying, and still followedby wet oxidization (pyrogenic oxidization) at 950° C., to therebysimultaneously grow the polycrystalline Si thermally-oxidized film 8 andthe SiN sideface thermally-oxidized film 13, respectively, i) on thesideface and upper part of the polycrystalline Si gate electrode 7 andii) on the sideface of the SiN film 11.

Herein, for improving reliability of the MIS structure including the ONOgate insulating film 9, there are defined three extremely essentialpoints:

1) First, the outer edge part of the SiN film 11 featuring highleakability which film is damaged by the above gate etching is removedby being converted to the SiN sideface thermally-oxidized film 13.

2) Second, the outer edge G of the polycrystalline Si gate electrode 7is moved back slightly inward relative to the outer edge N of the SiNfilm 11, to thereby relieve the gate electric field at the outer edge Nof the SiN film 11. For moving back the outer edge G of thepolycrystalline Si gate electrode 7, the polycrystalline Si'soxidization speed (gate electrode 7) faster than the SiN film'soxidization speed (SiN film 11) is used in the production method,according to the first embodiment of the present invention.

3) Third, adding the polycrystalline Si thermally-oxidized film 8 andthe SiN sideface thermally-oxidized film 13 establishes the structurewhere the ONO gate insulating film 9 localized below the gate electrode7 is sealed with and protected by the thermally stable material, thatis, the polycrystalline Si, the SiC and the thermally-oxidized film.

The thus established structure is essential for inhibiting the ONO gateinsulating film 9 from being deteriorated, which deterioration may becaused by an interaction with peripheral members or with the environmentin the subsequent high temperature contact annealing (1000° C., 2 min.)and the like.

Operation (f): After the forming of the polycrystalline Sithermally-oxidized film 8 and the SiN sideface thermally-oxidized film13, the ILD film 14 is deposited on the entire surface of the SiCepitaxial substrate 1 {see FIG. 5( f)}. Proper materials for the ILDfilm 14 include i) SiO₂ film (about 1 μm in thickness) deposited by anatmospheric pressure CVD using silane and oxygen as raw materials andii) phosphosilicate glass (PSG) which is i) above added by phosphor (P),but not limited to the above. Other materials capable of withstandingthe subsequent various heat treatment operations can be used for the ILDfilm 14. Then, the SiC epitaxial substrate 1 is put in an ordinarydiffusion furnace, followed by a calm heat treatment in an N₂ atmospherefor several 10 min., to thereby increase density of the ILD film 14. Theheat treatment temperature (for example, 900° C. to 1000° C.) for theabove operation is preferably lower than the temperature for forming theoxide silicon film 10.Operation (g): Then, the photoresist is applied to the entire surface ofthe SiC epitaxial substrate 1, then a post bake is sufficientlyimplemented, then volatile component of the photoresist is completelyevaporated, then the SiC epitaxial substrate 1 is dipped in the BHF acidsolution, then the second transient SiC thermally-oxidized film 202 {seeFIG. 5( f)} remaining on the backface of the SiC epitaxial substrate 1is completely removed, and then the BHF acid solution is cleaned offwith the super-pure water. A C face of the thus exposed backface of theSiC epitaxial substrate 1 is clean and free from damage or contaminant.The C face greatly contributes to decreasing resistance of the ohmiccontact.

Then, the SiC epitaxial substrate 1 wet with the super-pure water isdried, then immediately is set in a vacuum depositor kept at a highvacuum, and then a certain ohmic contact parent material is vacuumdeposited to the backface of the SiC epitaxial substrate 1. Examples ofthe ohmic contact parent material include Ni film having a thickness of50 nm to 100 nm, but not specifically limited thereto.

After the vacuum depositing of the ohmic contact parent material, theresist on the surface of the SiC epitaxial substrate 1 is completelystripped off using a special stripper solution, then the SiC epitaxialsubstrate 1 is sufficiently rinsed and dried, and then is immediatelyput in a rapid heat treatment device, to thereafter implement contactannealing at 1000° C. in a 100% high purity Ar atmosphere for 2 min.After the heat treatment, as shown in FIG. 5( g), the Ni film is alloyed(silicide) with the low resistance SiC substrate, to thereby form theohmic contact electrode 17 having an extremely low resistance of atleast 10⁻⁶ Ωcm² (10⁻⁶ Ωcm² to less than 10⁻⁵ Ωcm²).

Operation (h): Then, the photoresist mask (not shown) is formed on thesurface of the SiC epitaxial substrate 1 by the photolithography. Then,a photoresist as a protective film is applied to the entire backface ofthe SiC epitaxial substrate 1, followed by a sufficient drying, tothereafter open the gate contact window 15 in the ILD film 14 and thepolycrystalline Si thermally-oxidized film 8 (upper face part) byetching which uses the BHF acid solution. Forming of the protectingphotoresist on the backface of the SiC epitaxial substrate 1 can beomitted. However, due to the following roles, forming of the protectingphotoresist on the backface of the SiC epitaxial substrate 1 ispreferable:

1) Inhibiting the ohmic contact electrode 17 from disappearing or beingdeteriorated after being eluted to the BHF acid solution.

2) Inhibiting the ohmic contact material eluted from the backface of theSiC epitaxial substrate 1 from contaminating the surface of the SiCepitaxial substrate 1.

After the etching is ended, the photoresist mask is completely strippedoff by a special stripper solution, to thereby form the structure shownin FIG. 6( h).

Operation (i): Then, the SiC epitaxial substrate 1 is sufficientlycleaned and rinsed, followed by drying, and then is immediately put in ahigh vacuum magnetron spattering device. Then, a certain wiringmaterial, for example, Al having a thickness of 1 μm thickness is vacuumdeposited to the entire surface of the SiC epitaxial substrate 1.

Then, by the photolithography, the photoresist mask (not shown) isformed on the surface of the SiC epitaxial substrate 1 formed with theAl film, then again, a backface electrode protecting photoresist isapplied to the backface of the SiC epitaxial substrate 1, then the aboveresist is sufficiently dried, then the Al film is patterned using aphosphoric acid etching solution, to thereby form the inner wiring 16.The resist of the backface of the SiC epitaxial substrate 1 inhibitsdisappearing or altering of the ohmic contact electrode 17 on thebackface, which disappearing or altering may be caused by the ohmiccontact electrode 17's elution in the phosphoric acid etching solution.Without fear for the above disappearing or altering or when the Al filmis etched by the RIE, forming of resist of the backface of the SiCepitaxial substrate 1 can be omitted.

Finally, the resist mask and the backface electrode protecting resistare completely removed by a special stripper solution, followed by asufficient rinsing of the substrate and drying, to thereby form thefinal structure in FIG. 6( i). With the above operations, the siliconcarbide semiconductor device 90 having the MIS structure including theONO gate insulating film 9 is completed, according to the firstembodiment of the present invention.

FIG. 7 shows characteristics of the ONO gate insulating film 9 of thesemiconductor device 90, according to the first embodiment of thepresent invention. In FIG. 7, #ONO-1 and #ONO-2 are plots of Weibulldistribution, showing results of constant current stress TDDB (TimeDependent Dielectric Breakdown) test on the ONO gate insulating filmcapacitor (sample size=about 50) thus prepared. #ONO-1 has the oxidesilicon film 10 formed by the operation (c1), while #ONO-2 has the oxidesilicon film 10 formed by the operation (c3). Although not shown in FIG.7, the ONO gate insulating films 9, 9 formed by the operation (c2) andthe operation (c4) showed like test results. For comparison with thepresent invention, data (#ONO-0) disclosed by the present inventor basedon the conventional technology in the above non-patent document 5 isalso shown in FIG. 7.

FIG. 7 has the abscissa denoting electric charge density Q_(BD) (C/cm²)per unit area passing through the gate insulating film up to the TDDB(Time Dependent Dielectric Breakdown), and F in the ordinate denotescumulative failure rate. Q_(BD) denotes an essential index for measuringreliability corresponding to lifetime.

The test sample has the gate window 6 having an area (opening part) of3.14×10⁻⁴ cm², all the ONO gate insulating film 9 s have SiO₂ filmconversion film thickness about 40 nm, and the SiN film 11 and the SiNthermally-oxidized film 12 are respectively 53 nm and 5 nm in thickness.#ONO-1 and #ONO-2 under the present invention have a common heattreatment condition for the oxide silicon film 10, specifically, usingN₂O gas at 1275° C. for 20 min.

Compared with the test result of #ONO-0 of the conventional technology,the test results #ONO-1 and #ONO-2 according to the first embodiment ofthe present invention shift the TDDB lifetime to a longer lifetime side(higher Q_(BD)) while keeping an inclination of the Weibull distributioncurve. The above results denote that, compared with the conventionaltechnology, the TDDB lifetime according to the first embodiment isextended uniformly by a constant magnification without widening therange of distribution. From the graphs in FIG. 7, at the cumulativefailure rate F=50%, Q_(BD) (#ONO-0)=30 C/cm², Q_(BD) (#ONO-1)=64 C/cm²,and Q_(BD) (#ONO-2)=408 C/cm². Compared with the conventional technology(#ONO-0), the TDDB lifetime according to the first embodiment of thepresent invention is successfully improved to 2.1 times (=64/30, #ONO-1)to 13.6 times (=408/30, #ONO-2). It is known that the TDDB lifetime of athermally-oxidized film (thickness of 40 nm, polycrystalline Si gate) ona single crystal Si substrate is preferably Q_(BD) (#Si)=40 C/cm².Therefore, according to the first embodiment of the present invention,the TDDB lifetime of thermally-oxidized film on the single crystal Sisubstrate is successfully improved to 1.6 times (=64/40, ONO-1) to 10.2times (=408/40, #ONO-2).

As set forth above, according to the conventional technology (non-patentdocument 5), the TDDB lifetime of the ONO gate insulating film isgreatly improved to such an extent as to reach that of the Sithermally-oxidized film, but the ONO gate insulating film is notnecessarily sufficient for a long time operation at a temperature higherthan a practical upper limit temperature of the Si (MOS) device (thefirst problem), therefore, further requiring improvement in TDDBlifetime.

Moreover, the ONO gate insulating film according to the conventionaltechnology (non-patent document 5) is so structured as to form the firstoxide silicon film (which contacts SiC) with the SiC thermally-oxidizedfilm. Thereby, the high-reliability technology of the conventional gateinsulating film cannot be applied (second problem) to a specific MOS(MIS) structural device that is unable to use the SiC thermally-oxidizedfilm due to structural restriction and the like of the device. Examplesof the above specific MOS (MIS) structural device include a trench UMOSgate power transistor on 4H—SiC where the gate insulating film is to beformed on a plurality of crystalline faces having different thermaloxidizing speeds.

Under the present invention, the silicon carbide semiconductor device 90having the MOS structure and the production method therefor can solveany one or both of the first problem and second problem of theconventional technology (non-patent document 5) simultaneously.

<Effect>

As obvious from the explanation of the results in FIG. 7, the firstembodiment of the present invention solves the following first problemof the conventional technology (non-patent document 5):

The TDDB lifetime of the ONO gate insulating film is greatly improved tosuch an extent as to reach that of the Si thermally-oxidized film, butthe ONO gate insulating film is not necessarily sufficient for a longtime operation at a temperature higher than a practical upper limittemperature of the Si (MOS) device.

In addition, according to the first embodiment of the present invention,through any of the operation (c3) and the operation (c4), the oxidesilicon film 10 can be formed by an operation other than the thermaloxidizing of the SiC, in this case, also accomplishing the TDDB lifetimefar greater (by digit difference) than that of the conventionaltechnology. Namely, the first embodiment of the present invention solvesthe following second problem of the conventional technology (non-patentdocument 5):

The ONO gate insulating film according to the conventional technology(non-patent document 5) is so structured as to form the first oxidesilicon film (which contacts SiC) with the SiC thermally-oxidized film.Thereby, the high-reliability technology of the conventional gateinsulating film cannot be applied to a specific MOS (MIS) structuraldevice that is unable to use the SiC thermally-oxidized film due tostructural restriction and the like of the device.

As set forth above, the silicon carbide semiconductor device accordingto the first embodiment comprises: 1) a silicon carbide substrate 1; 2)a gate electrode 7 made of polycrystalline silicon; and 3) an ONOinsulating film 9 sandwiched between the silicon carbide substrate 1 andthe gate electrode 7 to thereby form a gate structure, the ONOinsulating film 9 including the followings formed sequentially from thesilicon carbide substrate 1: a) a first oxide silicon film 10, b) an SiNfilm 11, and c) an SiN thermally-oxidized film 12, wherein nitrogen isincluded in at least one of the following places: i) in the first oxidesilicon film 10 and in a vicinity of the silicon carbide substrate 1,and ii) in an interface between the silicon carbide substrate 1 and thefirst oxide silicon film 10.

The above structure can solve one of the first problem and the secondproblem of the conventional technology (non-patent document 5) or bothof the problems simultaneously. Namely, in terms of solving the firstproblem, the TDDB lifetime of the ONO gate insulating film 9 can befurther improved, and the ONO gate insulating film 9 can be operated fora long time at a temperature higher than a practical upper limittemperature of the Si (MOS) device. Moreover, in terms of solving thesecond problem, the high-reliability technology of the conventional gateinsulating film can be applied to a specific MOS (MIS) structural devicethat is unable to use the SiC thermally-oxidized film due to structuralrestriction and the like of the device, examples of the above specificMOS (MIS) structural device including a trench UMOS gate powertransistor on 4H—SiC where the gate insulating film is to be formed on aplurality of crystalline faces having different thermal oxidizingspeeds.

The first oxide silicon film 10 has a thickness from 3.5 nm to 25 nm,giving a preferable result to the above effect.

The first oxide silicon film 10 has the thickness from 4 nm to 10 nm,giving a more preferable result to the above effect.

The first oxide silicon film 10 is a non-SiC thermally-oxidized filmhaving an increased density, thus further improving reliability.

The silicon carbide semiconductor device 90 is an MOS capacitor, thusrealizing the MOS capacitor having the above effect.

In the method for producing the silicon carbide semiconductor device 90according to the first embodiment, the first oxide silicon film 10 isformed by a heat treatment in an oxidized nitrogen (NOx) gas atmospherein a period after forming of a precursor oxide silicon film and beforedepositing of the SiN film 11, see the operation (c1) and the operation(c3). Thereby, the oxide silicon film 10 including N in the SiCinterface or in the vicinity of the SiC interface can be formed easily,thus producing easily the silicon carbide semiconductor device 90 whichbrings about an effect such as improving the TDDB lifetime of the ONOgate insulating film 9.

In the method for producing the silicon carbide semiconductor device 90according to the first embodiment, the first oxide silicon film 10 isformed by a reoxidization in an oxidized nitrogen (NOx) gas atmospherein a period after forming of a precursor oxide silicon film and beforedepositing of the SiN film (N), see the operation (c1) and the operation(c3). Thereby, the oxide silicon film 10 including N in the SiCinterface or in the vicinity of the SiC interface can be formed easily,thus producing easily the silicon carbide semiconductor device 90 whichbrings about an effect such as improving the TDDB lifetime of the ONOgate insulating film 9.

In the method for producing the silicon carbide semiconductor device 90according to the first embodiment, the first oxide silicon film 10 isformed by thermally oxidizing a surface of the silicon carbide substrate1 in an oxidized nitrogen (NOx) gas atmosphere, see the operation (c2).Thereby, the oxide silicon film 10 including N in the SiC interface orin the vicinity of the SiC interface can be formed easily, thusproducing easily the silicon carbide semiconductor device 90 whichbrings about an effect such as improving the TDDB lifetime of the ONOgate insulating film 9.

In the method for producing the silicon carbide semiconductor device 90according to the first embodiment, the first oxide silicon film 10 isformed by the following sequential operations: 1) forming a thin oxidesilicon film by one of the following sub-operations: i) the operation(c1): a heat treatment in an oxidized nitrogen (NOx) gas atmosphere in aperiod after forming of a precursor oxide silicon film and beforedepositing of the SiN film 11, ii) the operation (c2): a reoxidizationin the oxidized nitrogen (NOx) gas atmosphere in the period afterforming of the precursor oxide silicon film and before depositing of theSiN film 11, and iii) the operation (c3): thermally oxidizing a surfaceof the silicon carbide substrate 1 in the oxidized nitrogen (NOx) gasatmosphere, and 2) the operation (c4): depositing, on the thin oxidesilicon film, another oxide silicon film which is formed by an operationother than the thermal oxidizing. Thereby, the oxide silicon film 10including N in the SiC interface or in the vicinity of the SiC interfacecan be formed easily, thus producing easily the silicon carbidesemiconductor device 90 which brings about an effect such as improvingthe TDDB lifetime of the ONO gate insulating film 9.

The oxidized nitrogen (NOx) gas atmosphere is formed by supplying anyone of the following gases: i) N₂O (nitrous oxide), ii) NO (nitrogenmonoxide), iii) NO₂ (nitrogen dioxide), iv) a mixture of at least two ofi) to iii), v) a diluted gas of any of i) to iii), and vi) a diluted gasof the mixture in iv). See the operation (c1) to the operation (c4).Thereby, the oxide silicon film 10 including N in the SiC interface orin the vicinity of the SiC interface can be formed easily, thusproducing easily the silicon carbide semiconductor device 90 whichbrings about an effect such as improving the TDDB lifetime of the ONOgate insulating film 9.

Any of the heat treatment, the reoxidization and the thermal oxidizationin the oxidized nitrogen (NOx) gas atmosphere is implemented in atemperature range of 1000° C. to 1400° C., see the operation (c1) to theoperation (c4). Thereby, the oxide silicon film 10 including N in theSiC interface or in the vicinity of the SiC interface can be formedeasily, thus producing easily the silicon carbide semiconductor device90 which brings about an effect such as improving the TDDB lifetime ofthe ONO gate insulating film 9.

Any of the heat treatment, the reoxidization and the thermal oxidizationin the oxidized nitrogen (NOx) gas atmosphere is implemented in thetemperature range of 1100° C. to 1350° C., see the operation (c1) to theoperation (c4). In view of shorter treatment time and lower treatmentdevice cost, 1100° C. to 1350° C. is practical and preferable.

The precursor oxide silicon film is formed by thermally oxidizing asurface of the silicon carbide substrate 1, see the operation (c1).Thereby, the oxide silicon film 10 including N in the SiC interface orin the vicinity of the SiC interface can be formed easily, thusproducing easily the silicon carbide semiconductor device 90 whichbrings about an effect such as improving the TDDB lifetime of the ONOgate insulating film 9.

The precursor oxide silicon film is formed by a depositing operationother than a thermal oxidizing, see the operation (c3). Thereby, theoxide silicon film 10 including N in the SiC interface or in thevicinity of the SiC interface can be formed easily, thus producingeasily the silicon carbide semiconductor device 90 which brings about aneffect such as improving the TDDB lifetime of the ONO gate insulatingfilm 9.

The depositing operation other than the thermal oxidizing is a chemicalvapor deposition, see the operation (c3). Thereby, the oxide siliconfilm 10 including N in the SiC interface or in the vicinity of the SiCinterface can be formed easily, thus producing easily the siliconcarbide semiconductor device 90 which brings about an effect such asimproving the TDDB lifetime of the ONO gate insulating film 9.

The precursor oxide silicon film is formed by thermally oxidizing any ofa polycrystalline silicon and an amorphous silicon film which aredeposited by a chemical vapor deposition, see the operation (c3).Thereby, the oxide silicon film 10 including N in the SiC interface orin the vicinity of the SiC interface can be formed easily, thusproducing easily the silicon carbide semiconductor device 90 whichbrings about an effect such as improving the TDDB lifetime of the ONOgate insulating film 9.

An operation after forming of the first oxide silicon film 10 isimplemented at a temperature less than or equal to a temperature of anyof the heat treatment, the reoxidization and the thermal oxidizing inthe oxidized nitrogen (NOx) gas atmosphere, see the operation (c1) tothe operation (c4). Thereby, the decreased TDDB lifetime or thedeteriorated interface between the oxide silicon film 10 and the SiC canbe inhibited.

Second Embodiment

According to the first embodiment, the ONO gate insulating film MISstructure (capacitor) having the field insulating film 3 on both sidesof a gate area is disclosed. The present invention is, however, notlimited to the MIS structure having the above field insulating film 3,and is applicable to a structure free from the field insulating film 3,bringing about the like effect.

<Structure>

FIG. 8 is a cross sectional view of an essential part of a siliconcarbide semiconductor device 90 having an MIS structure (capacitor)including a high-reliability ONO layered film, according to a secondembodiment of the present invention. Structural elements of the secondembodiment substantially same as those according to the first embedmentare denoted by the same reference numerals, and the explanationthereabout will be simplified or, as the case may be, omitted foravoiding prolixity.

There is provided the N⁺ type SiC epitaxial substrate 1 including anupper face having an n⁻ type epitaxial layer. At least on the sidefaceof the polycrystalline Si gate electrode 7, there is provided thepolycrystalline Si thermally-oxidized film 8 grown by thermal oxidizing.Between the SiC epitaxial substrate 1 and the polycrystalline Si gateelectrode 7, the ONO gate insulating film 9 having the 3-layer structureis sandwiched.

The lowermost part (SiC epitaxial substrate 1 side) of the 3-layerstructure of the ONO gate insulating film 9, namely, the thin oxidesilicon film 10 includes N (nitrogen) in at least one of the followingplaces: i) in an interface between the first oxide silicon film 10 andthe SiC epitaxial substrate 1 and ii) in the vicinity of the interface.The oxide silicon film 10 has a thickness from 3.5 nm to 25 nm,especially, thickness from 4 nm to 10 nm giving an extremely preferableresult.

Of the 3-layer structure of the ONO gate insulating film 9, the SiN film11 is the intermediate layer (═N) deposited by the LPCVD and the like,and the SiN thermally-oxidized film 12 grown by oxidizing the surface ofthe SiN film 11 is the uppermost layer (═O). Each of the SiN film 11(including the SiN sideface thermally-oxidized film 13) and the SiNthermally-oxidized film 12 is so formed as to share the outer edge withthe gate electrode 7 (including the polycrystalline Sithermally-oxidized film 8). The SiN film 11 and the SiNthermally-oxidized film 12 have thickness, for example, 53 nm and 5 nmrespectively. On the outer edge sideface of the SiN film 11, there isdisposed the thin SiN sideface thermally-oxidized film 13 (namely, SiO₂film) grown by thermally-oxidizing the SiN film 11. The SiN sidefacethermally-oxidized film 13 is an essential element for securingreliability of the ONO gate insulating film 9.

Moreover, the outer edge G of the polycrystalline Si gate electrode 7 isto be positioned inside the outer edge N of the SiN film 11. Failure ofmeeting the above positioning of the outer edge G will considerablydecrease reliability of the ONO gate insulating film 9, therefore, likethe first embodiment, positioning of both of the outer edge G and theouter edge N should be accurately controlled according to the secondembodiment.

On the gate electrode 7 and the oxide silicon film 10 (periphery of thegate electrode 7), the ILD film 14 is formed. The ILD film 14 is formedwith the gate contact window 15 opening through the gate electrode 7.

Via the gate contact window 15, the inner wiring 16 connects the gateelectrode 7 to other circuit element(s) on the same substrate or to theouter circuit.

On the backface of the SiC epitaxial substrate 1, there is disposed theohmic contact electrode 17 having an extremely low resistance. The ohmiccontact electrode 17 is formed by the following sequential operations:i) a contact metal such as Ni is vacuum deposited on the back of the SiCepitaxial substrate 1, ii) alloying the thus obtained with SiC by arapid heat treatment at a temperature lower than the thermal oxidizingtemperature for the oxide silicon film 10 (namely, SiCthermally-oxidized film) of the ONO gate insulating film 9, for example,1000° C. relative to the thermal oxidizing at 1100° C.

<Production Method>

Then, referring to FIG. 9( a) to FIG. 11( f), a method for producing theMIS structure (see FIG. 8) including the ONO gate insulating film 9 isto be explained, according to the second embodiment of the presentinvention.

Operation (a): A (0001) Si face 8° off cut n⁺ type 4H—SiC epitaxialsubstrate 1 having an upper surface where a high-quality n⁻ typeepitaxial layer is grown is sufficiently cleaned by the RCA cleaning andthe like. Then, after dry oxidizing, an SiC thermally-oxidized filmhaving a thickness about 10 nm is grown, thereafter immediately dippingthe SiC epitaxial substrate 1 in a buffered hydrofluoric acid solution(BHF=NH₄F+HF mix solution) and then removing the BHF acid solution.

The above sacrificial oxidization operation can inhibit, to a certainextent, i) a contaminant on the surface of the SiC epitaxial substrate 1or ii) a potential defect attributable to crystal imperfection fromentering the oxide silicon film 10. The SiC epitaxial substrate 1 afterthe sacrificial oxidization is again subjected to the RCA cleaning, andthen is dipped in the BHF acid solution for 5 sec. to 10 sec. so as toremove the chemically oxidized film generated on the surface of the SiCepitaxial substrate 1 at the final step of the cleaning. After the aboveoperation, the BHF acid solution is completely rinsed off with thesuper-pure water, to thereafter dry the SiC epitaxial substrate 1.

Immediately, by any of the operation (c1) to the operation (c4)according to the first embodiment, the oxide silicon film 10 is grown onthe entire surface of the SiC epitaxial substrate 1. Herein, the heattreatment temperature in the oxidized nitrogen (NOx) gas atmosphere isset higher than any other heat treatment temperature in the subsequentoperations. FIG. 9( a) shows a cross sectional structure of the MISstructure at this step in the operation.

In FIG. 9( a), the SiC epitaxial substrate 1 has the backface which isthe transient oxide silicon film 202 automatically formed in theoperation for forming the oxide silicon film 10. The transient oxidesilicon film 202 effectively removes the grinding damage layer on thebackface of the SiC epitaxial substrate 1, in addition, featuring anessential function to protect the backface of the SiC epitaxialsubstrate 1 from dry etching damage which may be caused when thepolycrystalline Si on the backface is removed, to be explained in thesubsequent operations.

Operation (b): After the forming of the oxide silicon film 10, the SiNfilm 11 (=second layer of the ONO gate insulating film 9) is depositedon the entire surface of the SiC epitaxial substrate 1 by the LPCVDusing SiH₂Cl₂ and O₂. Immediately after the depositing, the SiCepitaxial substrate 1 is subjected to the pyrogenic oxidization at 950°C., to thereby grow on the surface of the SiN film 11 the SiNthermally-oxidized film 12 (=third layer of the ONO gate insulating film9) having a certain thickness. FIG. 9( b) shows a cross sectionalstructure of the SiC epitaxial substrate 1 at this step in theoperation. The transient SiN film 203 and the transient SiNthermally-oxidized film 204 are automatically formed on the backface ofthe SiC epitaxial substrate 1 by, respectively, the depositing of theSiN film 11 and the growing of the SiN thermally-oxidized film 12.Operation (c): Then, by the low pressure CVD (growth temperature of 600°C. to 700° C.) using silane as raw material, the polycrystalline siliconfilm having a thickness of 300 nm to 400 nm is formed on the entiresurface and backface of the SiC epitaxial substrate 1. Then, by a knownheat diffusing method (treatment temperature 900° C. to 950° C.) usingphosphor chlorate (POCl₃) and oxygen, P (phosphor) having n typeimpurity is added to the polycrystalline silicon film, to thereby bringabout conductivity. Replacing the n type impurity hereinabove, a p typeimpurity can be added.

Then, a photoresist mask (not shown) is formed on the surface of the SiCepitaxial substrate 1 by photolithography, then, the polycrystalline Sifilm, the SiN thermally-oxidized film 12 and the SiN film 11 aresequentially etched by reactivity ion etching (RIE) using SF₆, tothereby substantially define (pre-defining) outer edges of i) thepolycrystalline Si gate electrode 7, ii) the SiN thermally-oxidized film12 and iii) the SiN film 11. With the above operation, a nonessentialpart of the ON layer (O: SiN thermally-oxidized film 12, N: SiN film 11)is etched (removed) accurately in a self-aligning manner by thephotoresist mask same as that for the polycrystalline Si gate electrode7, such that the ON layer can share an outer edge. At this step in theoperation, however, micron-level positional relation between the outeredge G of the polycrystalline Si gate electrode 7 and the outer edge Nof the SiN film 11 depends on the used RIE device or etchant gas and istherefore variable. The outer edge G of the polycrystalline Si gateelectrode 7 may be outside the outer edge N of the SiN film 11, or areverse positioning may occur.

There is an important reminder for the sequential etching operations,that is, to leave the oxide silicon film 10, in other words, notcompletely removing the oxide silicon film 10. An overetching tocompletely remove the oxide silicon film 10 allows a crystalline gratingdamage by plasma to enter the surface of the SiC epitaxial substrate 1thus exposed. With this, in the RIE of the SiN film 11, an etchant gashaving a high selectivity to the SiO₂ is to be used and an etching endpoint is to be accurately sensed, to thereby prevent the overetching.

After the sequential etching operations, the used resist is completelyremoved, then the back side of the SiC epitaxial substrate 1 issubjected to the dry etching while protecting the surface by applyingagain to the entire surface of the SiC epitaxial substrate 1 the resistmaterial (photoresist is allowable) having a thickness of more than orequal to 1 μm, then the transient polycrystalline Si film (includingthermally-oxidized film thereof, both not shown), the transient SiNthermally-oxidized film 204, the transient SiN film 203 {refer to FIG.9( b)} which are deposited on the back side are sequentially removed,and then the resist material for protecting the surface is stripped off,to thereby form the cross sectional structure in FIG. 10( c).

Operation (d): Then, the SiC epitaxial substrate 1 is again subjected tothe RCA cleaning, purifying-drying, followed by the wet oxidization(pyrogenic oxidization) at 950° C., to thereby simultaneously grow thepolycrystalline Si thermally-oxidized film 8 and the SiN sidefacethermally-oxidized film 13, respectively, i) on the sideface and upperpart of the polycrystalline Si gate electrode 7 and ii) on the sidefaceof the SiN film 11.

Herein, for improving reliability of the MIS structure including the ONOgate insulating film 9, there are three extremely essential points:

1) First, the outer edge part of the SiN film 11 featuring highleakability which film is damaged by the above gate etching is removedby being converted to the SiN sideface thermally-oxidized film 13.

2) Second, the outer edge G of the polycrystalline Si gate electrode 7is moved back slightly inward relative to the outer edge N of the SiNfilm 11, to thereby relieve the gate electric field at the outer edge Nof the SiN film 11.

For moving back the outer edge G of the polycrystalline Si gateelectrode 7, the polycrystalline Si's oxidization speed (gate electrode7) faster than the SiN film's oxidization speed (SiN film 11) is used inthe production method according to the second embodiment of the presentinvention.

3) Third, adding the polycrystalline Si thermally-oxidized film 8 andthe SiN sideface thermally-oxidized film 13 establishes the structurewhere the ONO gate insulating film 9 localized below the gate electrode7 is sealed with and protected by the thermally stable material, thatis, the polycrystalline Si, the SiC and thermally-oxidized film.

The thus established structure is essential for inhibiting the ONO gateinsulating film 9 from being deteriorated, which deterioration may becaused by an interaction with peripheral members or with the environmentin the subsequent high temperature contact annealing (1000° C., 2 min.)and the like.

Operation (e): After the forming of the polycrystalline Sithermally-oxidized film 8 and the SiN sideface thermally-oxidized film13, the ILD film 14 is deposited on the entire surface of the SiCepitaxial substrate 1 {see FIG. 11( e)}. Proper materials for the ILDfilm 14 include i) SiO₂ film (about 1 μm in thickness) deposited by anatmospheric pressure CVD using silane and oxygen as raw materials, andii) phosphosilicate glass (PSG) which is i) above added by phosphor (P),but not limited to the above. Other materials capable of withstandingthe subsequent various heat treatment operations can be used for the ILDfilm 14.

Then, the SiC epitaxial substrate 1 is put into an ordinary diffusionfurnace, followed by a calm heat treatment in an N₂ atmosphere forseveral 10 min., to thereby increase density of the ILD film 14. Theheat treatment temperature (for example, 900° C. to 1000° C.) in theabove operation is preferably lower than that for the oxide silicon film10.

Operation (f): Then, the photoresist is applied to the surface of theSiC epitaxial substrate 1, then a post bake is sufficiently implemented,then volatile component of the resist is completely evaporated, then theSiC epitaxial substrate 1 is dipped in the BHF acid solution, then thesecond transient oxide silicon film 202 {see FIG. 11( e)} remaining onthe backface of the SiC epitaxial substrate 1 is completely removed, andthen the BHF acid solution is cleaned off with the super-pure water. TheC face of the thus exposed backface of the SiC epitaxial substrate 1 isclean and free from damage or contaminant. The C face greatlycontributes to decreasing resistance of the ohmic contact.

Then, the SiC epitaxial substrate 1 wet with the super-pure water isdried, then immediately is set in the vacuum depositor kept at a highvacuum, and then a certain ohmic contact parent material is vacuumdeposited to the backface of the SiC epitaxial substrate 1. Examples ofthe ohmic contact parent material include Ni film having a thickness of50 nm to 100 nm, but not specifically limited thereto.

After the vacuum depositing of the ohmic contact parent material, theresist on the surface of the SiC epitaxial substrate 1 is completelystripped off with a special stripper solution, then the SiC epitaxialsubstrate 1 is sufficiently rinsed and dried, and then is immediatelyput in a rapid heat treatment device, to thereafter implement contactannealing at 1000° C. in a 100% high purity Ar atmosphere for 2 min.After the heat treatment, as shown in FIG. 11( f), the Ni film isalloyed (silicide) with the low resistance SiC substrate, to therebyform the ohmic contact electrode 17 having an extremely low resistanceof at least 10⁻⁶ Ωcm² (10⁻⁶ Ωcm² to less than 10⁻⁵ Ωcm²).

Operation (g): Hereinafter, by substantially the same operations asthose according to the first embodiment, the gate contact window 15 andthe inner wiring 16 are disposed on the SiC epitaxial substrate 1,according to the second embodiment. Then, the MIS structure includingthe ONO gate insulating film 9 shown in FIG. 8 is completed, accordingto the second embodiment of the present invention.<Effect>

The thus prepared MIS structure including the ONO gate insulating film 9according to the second embodiment showed as excellent TDDB lifetime asthat according to the first embodiment in FIG. 7. Namely, the secondembodiment of the present invention solves the following first problemof the conventional technology (non-patent document 5):

The TDDB lifetime of the ONO gate insulating film is greatly improved tosuch an extent as to reach that of the Si thermally-oxidized film, butthe ONO gate insulating film is not necessarily sufficient for a longtime operation at a temperature higher than a practical upper limittemperature of the Si (MOS) device.

In addition, according to the second embodiment of the presentinvention, through any of the operation (c3) combined with the (c4), theoxide silicon film 10 can be formed by an operation other than abovethermal oxidizing of the SiC, in this case, also accomplishing the TDDBlifetime far greater (by digit difference) than that of the conventionaltechnology. Namely, the second embodiment of the present inventionsolves the following second problem of the conventional technology(non-patent document 5):

The ONO gate insulating film according to the conventional technology(non-patent document 5) is so structured as to form the first oxidesilicon film (which contacts SiC) with the SiC thermally-oxidized film.Thereby, the high-reliability technology of the conventional gateinsulating film cannot be applied to a specific MOS (MIS) structuraldevice that is unable to use the SiC thermally-oxidized film due tostructural restriction and the like of the device.

Third Embodiment

According to a third embodiment of the present invention, a standard nchannel type planar power MOSFET cell is disclosed. Herein, the presentinvention is applicable to any cell forms including: square cell,hexagonal cell, circular cell, bipectinate (linear) cell and the like.

<Structure>

FIG. 12 is a cross sectional view of essential parts of a power MOSFETcell, according to the third embodiment of the present invention.

In FIG. 12, there is provided an n⁺ type single crystal SiC substrate100. On an upper surface (main face on upper side) of the n⁺ type singlecrystal SiC substrate 100, a first n⁻ type epitaxial layer 200 (10 μm inthickness) to which nitrogen (1×10¹⁶/cm³) is added is grownhomo-epitaxially. In FIG. 1 and FIG. 8 according to the respective firstembodiment and second embodiment, though not shown, an n⁻ type epitaxiallayer has a structure same as that of the first n⁻ type epitaxial layer200 in FIG. 12 according to the third embodiment. Substrates havingother structural systems such as 4H, 6H, 3C and 15R can be used, where Hdenotes hexagonal system, C denotes cubic system and R denotesrhombohedral system.

In a certain area of the surface layer of then n⁻ type epitaxial layer200, there are provided p type base areas 53 a, 53 b each having acertain depth and to which p type impurity is slighted added. In acertain area of the surface layer of each of the respective p type baseareas 53 a, 53 b, n⁺ type source areas 54 a, 54 b shallower than the ptype base areas 53 a, 53 b are formed apart from outer edge boundariesof the respective p type base areas 53 a, 53 b at a constant distance.On a substrate surface layer in the center of the p type base areas 53a, 53 b, a p⁺ type base contact area 57 shallower than the p type baseareas 53 a, 53 b are sandwiched between the n⁺ type source areas 54 a,54 b.

Selectively formed on the surface of the SiC substrate 100 are ONO gateinsulating films 9 a, 9 b. The ONO gate insulating films 9 a, 9 b eachhave 3-layer structure, having a sequential deposition, from lower side(the SiC substrate 100 side) thereof, including oxide silicon films 10a, 10 b, SiN films 11 a, 11 b and SiN thermally-oxidized films 12 a, 12b. Needless to say, the oxide silicon films 10 a, 10 b are thin andinclude N (nitrogen) in at least one of the following places: i) in aninterface between the oxide silicon films 10 a, 10 b and the SiCsubstrate 100 and ii) in the vicinity of the interface. The oxidesilicon films 10 a, 10 b each have a thickness from 3.5 nm to 25 nm,especially, thickness from 4 nm to 10 nm giving an extremely preferableresult. On sidefaces of the SiN films 11 a, 11 b, respectively, thereare disposed SiN sideface thermally-oxidized films 13 a, 13 b grown bythermally-oxidizing the SiN films 11 a, 11 b.

On the ONO gate insulating films 9 a, 9 b, respectively, there areprovided conductive gate electrodes 7 a, 7 b made of polycrystalline Si.At upper parts and on sidefaces of the polycrystalline Si gateelectrodes 7 a, 7 b, respectively, there are provided polycrystalline Sisideface thermally-oxidized films 8 a, 8 b.

On the SiC substrate 100 including the polycrystalline Si sidefacethermally-oxidized films 8 a, 8 b, there are formed ILD films 14 a, 14b. A source window 63 opened in the ILD films 14 a, 14 b passes throughthe n⁺ type source areas 54 a, 54 b and the p⁺ type base contact area57. At the base of the source window 63, there is formed a sourcecontact electrode 64. The source contact electrode 64 is formed by thefollowing sequential operations: i) a thin metal film parent materialsuch as Ni is selectively disposed at the base of the source window 63,and ii) alloying the thus obtained with SiC by a rapid heat treatment.The source contact electrode 64 realizes an ohmic contact simultaneouslywith the n⁺ type source areas 54 a, 54 b and the p⁺ type base contactarea 57. On a backface of the SiC substrate 100, there is a drainelectrode 17 formed by an operation like the operation for forming thesource contact electrode 64. Via the source window 63, the inner wiring16 connects the source contact electrode 64 to other circuit element(s)on the same substrate or to the outer circuit.

<Production Method>

Then, referring to FIG. 13( a) to FIG. 16( h), a method for producingthe planar type power MOSFET cell is to be explained, according to thethird embodiment of the present invention.

Operation (a): At first, the n⁺ type SiC epitaxial substrate 100 isprepared having the main face on which then⁻ type epitaxial layer 200 isgrown homo-epitaxially, and then a CVD oxide film 20 having a thicknessof 20 nm to 30 nm is deposited on the surface of then⁻ type epitaxiallayer 200. Then, on the thus obtained, a polycrystalline Si film(thickness about 1.5 μm) as an ion implantation mask is formed by theLPCVD (low pressure chemical vapor deposition). Other than thepolycrystalline Si film, any of SiO₂ film and PSG (phosphosilicateglass) film which are formed by the CVD may be used. The CVD oxide film20 can be omitted. The CVD oxide film 20, is however, preferred to beused due to its useful effects and functions (see below) when thepolycrystalline Si film is used as the ion implantation mask:

(1) Working as a protective film for preventing the polycrystalline Sifilm from causing an unexpected reaction with then n⁻ type epitaxiallayer 200,

(2) Sensing an end point for anisotropy etching of the polycrystallineSi mask and working as an etching stopper film,

(3) Working as a surface protective film in ion implantation of the ptype base impurity.

Then, using anisotropy etching operations such as photolithography,reactivity ion etching (RIE) and the like, the polycrystalline Si filmabove an area where the p type base areas 53 a, 53 b are to be formed isvertically removed, to thereafter form first ion implantation masks 21a, 21 b. Etchant gases such as SF₆ used for the RIE of thepolycrystalline Si film enables i) a high selectivity etching to thethermally-oxidized film and ii) an end point sensing, preventing theplasma damage to the surface of the SiC substrate 100, especially, tothe channel area.

Then, p type impurity ion is implanted, to thereby form the p type baseareas 53 a, 53 b, as shown in FIG. 13( a). Actually, the polycrystallineSi film is adhered to the backface of the SiC substrate 100, however,not shown in FIG. 13( a). An example of the selective ion implantationcondition for the p type base areas 53 a, 53 b includes:

Impurity: Al⁺ ion

Substrate temperature: 750° C.

Acceleration voltage/dose: 360 keV/5×10⁻¹³ cm⁻²

After the ion implantation of the p type base, the CVD oxide film 20 andthe first ion implantation masks 21 a, 21 b are removed by the wetetching. Operation (b): Then, the n⁺ type source areas 54 a, 54 b andthe p⁺ type base contact area 57 are formed, as shown in FIG. 13( b), byimplementing substantially the same operations as those for theselective ion implantation of the p type base areas 53 a, 53 b.

Examples of the selective ion implantation condition for the n⁺ typesource areas 54 a, 54 b include:

Impurity: P⁺ ion

Substrate temperature: 500° C.

Acceleration voltage/dose: 160 keV/2.0×10¹⁵ cm⁻²

-   -   100 keV/1.0×10¹⁵ cm⁻²    -   70 keV/6.0×10¹⁴ cm⁻²    -   40 keV/5.0×10¹⁴ cm⁻²

Moreover, an example of the selective ion implantation condition for thep⁺ type base contact area 57 includes:

Impurity: Al⁺ ion

Substrate temperature: 750° C.

Acceleration voltage/dose: 100 keV/3.0×10¹⁵ cm⁻²

-   -   70 keV/2.0×10¹⁵ cm⁻²    -   50 keV/1.0×10¹⁵ cm⁻²    -   30 keV/1.0×10¹⁵ cm⁻²

After all the ion implantation operations ended, the SiC substrate 100is dipped in a mix solution of hydrofluoric acid and nitric acid,completely removing i) all the masks used and ii) unnecessary maskmaterials adhered to the backface of the SiC substrate 100. For removingthe mask, the SiC substrate 100 can be alternately dipped in heatphosphoric acid solution and BHF solution, to thereby sequentiallyremove the polycrystalline Si film and the SiO₂ film.

Then, the SiC substrate 100 with the mask removed is cleaned and dried,followed by a heat treatment at 1700° C. for 1 min. in a high-purityatmospheric pressure Ar atmosphere, to thereby activate, once for all,all the conductive impurities that are ion-implanted on the p type baseareas 53 a, 53 b, the n⁺ type source areas 54 a, 54 b and the p⁺ typebase contact area 57.

Operation (c): Then, the SiC substrate 100 sufficiently cleaned by theRCA cleaning and the like is thermally-oxidized in a dry oxygenatmosphere, to thereby grow the thermally-oxidized film on the surfaceand backface of the SiC substrate 100, then, immediately remove the filmwith the BHF acid solution. The sacrificial oxide film has a thicknessless than 50 nm, preferably 5 nm to 20 nm. The SiC substrate 100 afterthe sacrificial oxidization is, again, sufficiently cleaned by the RCAcleaning. Then, a thick insulating film is formed on the surface of theSiC substrate 100 by the thermal oxidizing, the CVD and the like, tothereby form, by i) a known photolithography and ii) a known dry or wetetching, 1) a field area (not shown) having a thick oxide film and 2) anelement area (unit cell) 70 (refer to FIG. 12) free from the thick oxidefilm. Hereinabove, at this step in the operation, the element area 70 issubstantially the same in shape as that in FIG. 13( b), though leaving adifference in that an outer periphery of the element area 70 is formedwith the field area (not shown).

Then, the SiC substrate 100 is, again, sufficiently cleaned by the RCAcleaning and the like. For removing the chemically oxidized film (SiO₂)formed on the surface of the element area 70 at the final step of thecleaning, the element area 70 (the SiC substrate 100) is dipped in adilute hydrofluoric acid solution for 5 sec. to 10 sec., followed by acomplete rinsing off of the dilute hydrofluoric acid solution with thesuper-pure water, and still followed by drying. Immediately after that,the oxide silicon films 10 a, 10 b, namely, the first layers of the ONOgate insulating films 9 a, 9 b are formed on the surface of the SiCsubstrate 100 of the element area 70. For forming the oxide siliconfilms 10 a, 10 b, any of the operation (c1) to the operation (c4)according to the first embodiment can be arbitrarily used.

Then, the SiN films 11 a, 11 b (second layers) are deposited by theLPCVD on the oxide silicon films 10 a, 10 b. Finally, the SiN films 11a, 11 b are thermally-oxidized (for example, pyrogenic oxidization) forgrowing the SiN thermally-oxidized films 12 a, 12 b (third layer) on thesurface, to thereby obtain the structure in FIG. 13( c). A film havingthe ONO structure is formed also on the backface of the epitaxialsubstrate 100, however, not shown in FIG. 13( c). Conditions for formingeach of the films according to the third embodiment can be substantiallythe same as those according to the first embodiment and the secondembodiment of the present invention.

Herein, an essential point is that the NOx heat treatment temperaturefor the oxide silicon films 10 a, 10 b should be set higher than any ofheat treatment temperatures in the subsequent operations. A rapid heattreatment is implemented hereinafter at 1000° C. for realizing an ohmiccontact between the source contact electrode 64 on the surface and thedrain electrode 17 on the backface. Therefore, a higher temperature, forexample, 1275° C. can be selected for the heat treatment of the oxidesilicon films 10 a, 10 b.

Operation (d): Then, on the entire surface and backface of the SiCsubstrate 100, a polycrystalline Si film having a thickness of 300 nm to400 nm is formed by a low pressure CVD (growth temperature of 600° C. to700° C.) using silane as raw material.

Then, an n type impurity P (phosphor) is added to the polycrystalline Sifilm by a known heat diffusing method (treatment temperature 900° C. to950° C.) using phosphor chlorate (POCl₃) and oxygen, to thereby bringabout conductivity. A p type impurity such as B (boron) and the like canbe added, replacing the P (phosphor).

Then, by the photolithography and the reactivity ion etching (RIE)(which uses C₂F₆ and oxygen as etchants), unnecessary parts aresequentially removed from i) the polycrystalline Si film on the surfaceof the epitaxial substrate 100 side, and ii) the SiN thermally-oxidizedfilms 12 a, 12 b and SiN films 11 a, 11 b of the ONO gate insulatingfilms 9 a, 9 b. Then, removing the photoresist brings about thestructure in FIG. 14( d). In this operation, the gate electrodes 7 a, 7b are defined (position specified). Herein, the polycrystalline Si filmis formed also on the backface of the epitaxial substrate 100, however,not shown in FIG. 14( d).

Operation (e): Then, after the RIE, the SiC epitaxial substrate 100 issubjected to the RCA cleaning, followed by purifying-drying and the wetoxidization (pyrogenic oxidization) at 950° C., to therebysimultaneously grow the polycrystalline Si thermally-oxidized films 8 a,8 b and the SiN sideface thermally-oxidized films 13 a, 13 b,respectively, i) on the sideface and upper part of the polycrystallineSi gate electrodes 7 a, 7 b and ii) on the sideface of the SiN films 11a, 11 b, as shown in FIG. 14( e).

In this operation (e), the sideface at the outer edge part of theleak-current SiN film which sideface is damaged by the gate etching inthe operation (d) is removed by converting the sideface to thethermally-oxidized films 13 a, 13 b, and as shown in FIG. 14( e), theouter edge G of each of the polycrystalline Si gate electrodes 7 a, 7 bis moved back slightly inward relative to the outer edge N of each ofthe SiN films 11 a, 11 b, to thereby relieve the gate electric field atthe outer edge N of each of the SiN films 11 a, 11 b, thus improvingreliability. For moving back the outer edge G of each of thepolycrystalline Si gate electrodes 7 a, 7 b, the polycrystalline Si'soxidization speed (gate electrodes 7 a, 7 b) faster than the SiN film'soxidization speed (SiN films 11 a, 11 b) is used in the productionmethod, according to the third embodiment of the present invention.

Moreover, in this operation (e), adding the polycrystalline Sithermally-oxidized films 8 a, 8 b and the SiN sidefacethermally-oxidized films 13 a, 13 b establishes a structure where theONO gate insulating films 9 a, 9 b localized below the gate electrodes 7a, 7 b respectively are sealed with and protected by the thermallystable material, that is, the polycrystalline Si, the SiC and thethermally-oxidized film. The thus established structure is essential forinhibiting the ONO gate insulating films 9 a, 9 b from beingdeteriorated, which deterioration may be caused by the interaction withthe peripheral members or with the environment in the subsequent hightemperature contact annealing (1000° C., 2 min.) and the like. Herein,the polycrystalline Si thermally-oxidized films 8 a, 8 b can be formedat the upper part of the gate electrodes 7 a, 7 b (not only the sidefacethereof), decreasing thickness of the polycrystalline Si gate electrodes7 a, 7 b. In view of the thus decreased thickness, an initial thicknessof the crystal Si gate electrodes 7 a, 7 b is specified.

Operation (f): Then, as shown in FIG. 15( f), the ILD film 14 isdeposited on the entire surface of the SiC substrate 100. Propermaterials for the ILD film 14 include i) SiO₂ film (=NSG, about 1 μm inthickness) deposited by an atmospheric pressure CVD using silane andoxygen as raw materials, ii) phosphosilicate glass (PSG) which is i)above added by phosphor (P) and iii) boron phosphosilicate glass (BPSG)which is ii) above added by boron, but not limited to the above.

Then, the SiC substrate 100 is put in an ordinary diffusion furnace,followed by a calm heat treatment in an N₂ atmosphere for several 10min., to thereby increase density of the ILD film 14. The heat treatmenttemperature (for example 900° C. to 1000° C.) for the above operation ispreferably lower than the temperature for forming (thermally oxidizing)the oxide silicon films 10 a, 10 b.

Operation (g): Then, by i) a known photolithography and ii) a known dryor wet etching, the source window 63 is opened a) in the ILD film 14 onthe surface of the SiC substrate 100 side and b) in the oxide siliconfilms 10 a, 10 b each of which is the SiC thermally-oxidized film of theONO gate insulating film 9. A gate contact window (not shown) formedaround the element area 70 in FIG. 12 is opened simultaneously with theabove operation. When the etchant solution or gas goes to the back ofthe SiC substrate 100, the thermally-oxidized film (not shown) on thetransient polycrystalline Si film on the backface is also simultaneouslyremoved.

After the etching of the ILD film 14 and oxide silicon films 10 a, 10 b,a source contact electrode parent material 25 is vacuum deposited, by afilm-forming operation such as DC (direct current) spattering and thelike, on the entire surface of the SiC substrate 100 where thephotoresist-etching mask remains. Ni film or CO film having a thickness,for example, 50 nm can be used for the source contact electrode parentmaterial 25.

After the vacuum depositing of the source contact electrode parentmaterial 25, the SiC substrate 100 is dipped in a special photoresiststripper, to thereby completely remove the photoresist remaining on thesurface of the SiC substrate 100. With this, as shown in FIG. 15( g), asubstrate structure is formed where the source contact electrode parentmaterial 25 is deposited only on the source window 63 and at the base ofthe gate contact window (drawing lines and signs are not shown).

Operation (h): Then, the SiC substrate 100 is sufficiently rinsed anddried, then a protective resist material (photoresist is allowable)having a thickness of more than or equal to 1 μm is applied to theentire surface of the SiC substrate 100, to thereafter sequentiallyremove, by dry etching, the polycrystalline silicon film, the SiNthermally-oxidized film and the SiN film (not shown) which are remainingon the backface of the SiC substrate 100 side. The above protectiveresist is used for inhibiting the source contact electrode parentmaterial 25 and the gate insulating films 10 a, 10 b from beingdeteriorated, which deterioration may be caused by plasma damage,charging or contaminant in the dry etching.

Then, the SiC substrate 100 is dipped in the BHF acid solution, tothereby remove a transient SiC thermally-oxidized film (not shown)formed in the forming of the oxide silicon films 10 a, 10 b, thusexposing a pure crystalline face on the backface of the epitaxialsubstrate 100. The BHF acid solution is completely rinsed off with thesuper-pure water, followed by drying, then the SiC substrate 100 isimmediately set in a vacuum depositor kept at high vacuum, to therebyvacuum deposit a certain drain contact electrode parent material (notshown) on the backface. Examples of the certain drain contact electrodeparent material on the backface includes Ni film or CO film having athickness of 50 nm to 100 nm.

Then, the resist used for protecting the surface is completely strippedoff with a special stripper solution, then the epitaxial substrate 100is sufficiently cleaned, rinsed and dried, then the epitaxial substrate100 is immediately set at a rapid heat treatment device, and then issubjected to a rapid heat treatment (contact annealing) in a high-purityAr atmosphere at 1000° C. for 2 min. By the above heat treatment, thecontact electrode parent materials (Ni film) deposited on i) the base ofthe source window 63 {see the source contact electrode parent material25 in FIG. 15( g)}, ii) the base of the gate contact window (not shown)and iii) the backface of the gate contact window (not shown) are alloyedrespectively with I) the n⁺ type source areas 54 a, 54 b (/p⁺ type basecontact area 57) {see FIG. 13( b)}, II} the polycrystalline Si gateelectrode contact area (not shown), and III) the backface of the n⁺ typeSiC epitaxial substrate 100, thus respectively forming A) the sourcecontact electrode 64, B) the gate electrode (not shown) and C) the drainelectrode 17 which three causing an ohmic contact featuring an extremelylow resistance, to thereby form the substrate structure in FIG. 16( h).

Operation (i): Finally, the SiC substrate 100 after the contactannealing is set at a magnetron spattering device kept in a high vacuum,then a certain wiring material, for example, Al film is vacuumdeposited, by a thickness of 3 μm, to the entire surface of the SiCsubstrate 100.

Then, the photoresist mask is formed, by the photolithography, on theupper face of the SiC substrate 100 formed with the Al film, then abackface electrode protecting photoresist is applied to the backface ofthe SiC substrate 100, then this photoresist is sufficiently dried, andthen the Al film is patterned by the RIE, to thereby form an innerwiring (not shown) connecting to the inner wiring 16 (connecting to thesource contact electrode 64) and to the gate electrode contact, as shownin FIG. 12.

Finally, the resist mask is completely removed with the special strippersolution, and the SiC substrate 100 is sufficiently rinsed and dried,thus completing the planar type power MOSFET cell, according to thethird embodiment of the present invention shown in FIG. 12.

<Effect>

The thus prepared planar type power MOSFET cell according to the thirdembodiment of the present invention has the MIS structure including theONO gate insulating films 9 a, 9 b, showing transistor characteristicmore excellent than that of the planar type power MOSFET cell having anordinary simple SiC thermally-oxidized gate oxide film.

Part of the MIS structure including the ONO gate insulating films 9 a, 9b according to the third embodiment showed as high TDDB lifetimedistribution (refer to FIG. 7) as that according to the firstembodiment. Namely, the MIS structure planar type power MOSFET cellincluding the ONO gate insulating films 9 a, 9 b and the productionmethod therefor according to the third embodiment of the presentinvention have solved the following first problem of the conventionalplanar type power MOSFET of the conventional technology (non-patentdocument 5):

The TDDB lifetime of the ONO gate insulating film is greatly improved tosuch an extent as to reach that of the Si thermally-oxidized film, butthe ONO gate insulating film is not necessarily sufficient for a longtime operation at a temperature higher than a practical upper limittemperature of the Si (MOS) device.

In addition, according to the third embodiment of the present invention,through any of the operation (c3) and the operation (c4), the oxidesilicon films 10 a, 10 b are formed by an operation other than thethermal oxidizing of the SiC. Namely, the third embodiment of thepresent invention solves the following second problem of theconventional technology (non-patent document 5):

The ONO gate insulating film according to the conventional technology(non-patent document 5) is so structured as to form the first oxidesilicon film (which contacts SiC) with the SiC thermally-oxidized film.Thereby, the high-reliability technology of the conventional gateinsulating film cannot be applied to a specific MOS (MIS) structuraldevice that is unable to use the SiC thermally-oxidized film due tostructural restriction and the like of the device.

The third embodiment is extremely effective for producing the powerUMOSFET having the ONO gate insulating film structure formed at thetrench of the SiC substrate.

Fourth Embodiment

According to the third embodiment, the ONO gate insulating filmstructure of the present invention is applied to the planar type powerMOSFET cell. The present invention is, however, not limited to theabove. The ONO gate insulating film structure is also applicable to anIGBT (Insulated Gate Bipolar Transistor) cell having a like elementstructure, according to a fourth embodiment of the present invention.According to the fourth embodiment, substantially the same effect can bebrought about as that brought about by the planar type power MOSFET cellaccording to the third embodiment.

The first embodiment to the fourth embodiment above are for a goodunderstanding of the present invention, and therefore, not limiting thepresent invention. Therefore, the elements disclosed in the above fourembodiments may include all design changes or equivalent thereof withinthe technical range of the present invention.

Moreover, the claimed structural elements are to be set forth relativeto the structural elements according to the embodiments. Specifically,the SiC epitaxial substrate 1 (the SiC substrate 100 and the epitaxiallayer 200 in FIG. 12) according to the embodiments corresponds to theclaimed silicon carbide substrate, the oxide silicon films 10, 10 a, 10b according to the embodiments correspond to the claimed first oxidesilicon film, the SiN films 11, 11 a, 1 lb correspond to the claimedsilicon nitride film, the SiN thermally-oxidized film 12 according tothe embodiments corresponds to the claimed silicon nitridethermally-oxidized film, and the ONO gate insulating film 9 according tothe embodiments corresponds to the claimed ONO insulating film.

This application is based on a prior Japanese Patent Application No.2005-247175 (filed on Aug. 29, 2005 in Japan). The entire contents ofthe Japanese Patent Application No. 2005-247175 from which priority isclaimed are incorporated herein by reference, in order to take someprotection against translation errors or omitted portions.

The scope of the present invention is defined with reference to thefollowing claims.

INDUSTRIAL APPLICABILITY

The present invention can provide a silicon carbide semiconductor devicehaving an improved practical upper limit temperature and a productionmethod for the device.

The invention claimed is:
 1. A method for producing a silicon carbidesemiconductor device, the silicon carbide semiconductor devicecomprising: a silicon carbide substrate; a gate electrode made ofpolycrystalline silicon; and an ONO insulating film sandwiched betweenthe silicon carbide substrate and the gate electrode to thereby form agate structure, the ONO insulating film including the followings formedsequentially from the silicon carbide substrate: a first oxide siliconfilm (O), an SiN film (N), and an SiN thermally-oxidized film (O), themethod comprising forming the first oxide silicon film (O) by areoxidization in an oxidized nitrogen (NOx) gas atmosphere in a periodafter forming of a precursor oxide silicon film and before depositing ofthe SiN film (N).
 2. The method for producing the silicon carbidesemiconductor device as claimed in claim 1, wherein the oxidizednitrogen (NOx) gas atmosphere is formed by supplying any one of thefollowing gases: i) N₂O (nitrous oxide), ii) NO (nitrogen monoxide),iii) NO₂ (nitrogen dioxide), iv) a mixture of at least two of i) toiii), v) a diluted gas of any of i) to iii), and vi) a diluted gas ofthe mixture in iv).
 3. The method for producing the silicon carbidesemiconductor device as claimed in claim 1, wherein the reoxidization inthe oxidized nitrogen (NOx) gas atmosphere is implemented in atemperature range of 1000° C. to 1400° C.
 4. The method for producingthe silicon carbide semiconductor device as claimed in claim 1, whereinthe precursor oxide silicon film is formed by thermally oxidizing asurface of the silicon carbide substrate.
 5. The method for producingthe silicon carbide semiconductor device as claimed in claim 1, whereinthe precursor oxide silicon film is formed by a depositing operationother than a thermal oxidizing.
 6. The method for producing the siliconcarbide semiconductor device as claimed in claim 5, wherein thedepositing operation other than the thermal oxidizing is a chemicalvapor deposition.
 7. The method for producing the silicon carbidesemiconductor device as claimed in claim 1, wherein the precursor oxidesilicon film is formed by thermally oxidizing any of a polycrystallinesilicon and an amorphous silicon film which are deposited by a chemicalvapor deposition.
 8. The method for producing the silicon carbidesemiconductor device as claimed in claim 1, wherein an operation afterforming of the first oxide silicon film (O) is implemented at atemperature less than or equal to a temperature of the reoxidization inthe oxidized nitrogen (NOx) gas atmosphere.
 9. The method for producingthe silicon carbide semiconductor device as claimed in claim 3, whereinthe reoxidization in the oxidized nitrogen (NOx) gas atmosphere isimplemented in the temperature range of 1100° C. to 1350° C.